The interference fringe of the reflection hologram is different from the rainbow hologram, a majority of which is parallel to the recording medium surface and thus the duplication of images cannot be achieved by the mechanical means, the duplication of images can be achieved only by the optical method. The exhibition of a holographic image is generally achieved by the contrast of the interference fringe. The material for recording the reflection hologram is various, but only fewer is ideal, even less for the scale production. Silver salt and dichromated gelatin as well as photopolymeric material disclosed in the U.S. Pat. No. 3,658,526 are mostly used.
The silver salt has high sensitivity, its exhibition of interference fringe is achieved by the bright-dark contrast of the material after photo-sensitization, but the diffraction efficiency is low, it is only about 40% even if a method of dilution development is adopted.
Dichromated gelatin is currently used as the common material for preparing the reflection hologram, the interference fringe thereof is achieved by the difference of the reflective index, the holographic image produced by dichromated gelatin has high diffraction efficiency as more than 85%, therefore many hologram elements are made of this material. However, it has a lot of shortages, such as low photosensitivity, a short storage life, and photographic plate being currently manufactured for use; at the same time, this material needs wet processing after imaging, the holographic image is affected greatly by the environment and can be easily image-erased in the environment with high moisture.
As to the photopolymeric material, such as the photopolymer in the material disclosed in the U.S. Pat. No. 3,658,526, the exhibition of the interference fringe is achieved by the different reflective indexes of two polymers. Although it can overcome the shortcomings of the silver salt and the dichromated gelatin, it only has the limited visual response to the visible light, which is affected by the resolution, therefore it is limited to be used for the transmission hologram, and, when being used for the reflection hologram, the reflection efficiency is very low.